Hydraulic drive mechanism



Dec. 22, 1959 Filed Feb. 4, 1955 W. M. SHAFFER HYDRAULIC DR IVE MECHANISM 5 Sheets-Sheet 1 INVENTOR. WAL 752 SHAFFEE BY 2/015: WA rrs, EDGEE TON z MNENN v A T 7' GENE Y5 Dec. 22, 1959 Filed Feb. 4, 1955 Dec. 22, 1959 k w. M. SHAFFER 2,917,897

v HYDRAULIC DRIVE MECHANISM Filed Feb. 4, 1955 5 Sheets-Sheet 3 INVENTOR. WALI'EB M. SHAFFEE BIZ/BY, wan-s, saesgrau n MENENNY ATTORNEYS Dec. 22, 1959 w. M. SHAFFER HYDRAULIC DRIVE MECHANISM 5 Sheets-Sheet 4 Filed Feb. 4, 1955 EV mm NMWHM fl nuwfiw mmuww'fifi .m l I I- I A 5 m@ Q Ma m N v M R W m. R m n A PICHEKWATTS,

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MN MW V 17,897 a @HYDRAULIGDRIVEMECHAMSMQ walter M;shairego eie anayohie :Application Februaryt4, 1955,-serte1.iwo-.as6, s6 i 11:0 am retreat Another object of thistinvention is toiprovidexashydraulic drive for a lift truck which has directional, speed ;and creeping control obtained by valves in afiuidsystem. A .still'further object of the: invention is (to-provide a'thydr'aulic drive. forilift trucks in 'which. automatic .espeed shifting based upon: torque 'requirements at the .motonoutput' shaft is obtaned. f LAnother. object of this invention-is to -providea :hygdraulic drive .for alift truck which has .a manual overa-ride for an automatic speed shifting system based on etorqueirequirement at the motor ,output; shaft.

.' Another; important object :of the invention is toaprovide a hydraul-ic'drivesystem for ,atlift;truck-in which .a :plurality ofjpumps driven -by the motor -isused, the

gpurnps being operated singly or in combination to ob- ,ppump supplies oil ,under .pressurefltoa m'otor which is econne'cted to the lift truck drive mechanism. *Therpum'p -is sized to provide sufi'icient' oil forlow-speed; operation 153.1 ahigh torque requirement-or for conditions where low speed is i desired in rnaneuvering the vehiele. Ai second ,:purnp-is automatically incorporated in theysystem when :higher speed isdesired requiring a largerQsupply of oil 7 ;t o,=,the motor. -Whe'n the motor speed is no longer .re-

quired a second :pump is lu'nloaded and ith e 'first apump imaintains the pressure in-the-system. Aqtthirdmu'mp is automatically-loaded for (supplying additional oil to the motor when high speed goperationof' the vehicle is required.

The combined, displacement of the three pumps -.is aequal'to-the displacement of the motjor therebyproducing a motor :shaft speed equal tozthe 'lift truck engine speed ginthe high speed range. v

The hydraulic lift system is connected to the pumps wutilized for driving purposes thereby avoiding duplicaqti'on and-increasing. the overall efficiency of .the truck. sufilcient --capacity;. is; available for operating the lifting 'cylinder in the :speed ranges 'o'ther than'the highspeed range :since :there would be the capacity of at least one -.pump-available @foroperating the. lift, and two :pumps ava'ilable foryproducing motion-of the .vehicle. By utiltizingthis :pumping arrangement 'certainadvanta-ges-of spacexequirements are obtained by locating the pumps fat the of the engine and thereby eliminating .the p' ump usnally located at thefan end of the engine. This resli na shorter drive unit and frees space which ina'y used for additional "c'ounterweight or other purposes.

I StatesPatent -C Pa ented Dec. 22, 1959 The control of-the system is :based generally on main- "-tfairi'ing a' predetermined pressure Y in the supply 1 system. -I f the speed l 'ofthe' vehicle which is reflected in the-dis- 'placem'ent of the-motor is greater than the displacement 5 of thef pun'ip which is in ope'ration, thesupply -pres'su re Wu1d be low and--thusactuate a control valve' which would cause another pump to begin operating and refstore the supplypressure to normal. Since this control -is automatiq the operator'of-the'vehicle is relieved of '=the necessity-of manual control of-the number of pumps whi'ch are"operating. :However, manual override of the *automatic control is provided when it is desired to {operate the drive with one pump only even though' the tor'que requirement is low.

*Acordingly, it is an important object of this -inve'ntion to-provide a-lifttruck having ahyd'raulic -drive -and -1ift system-wherein'a plurality of'pumps are-available-' foroperating both the lift and the drive system-inidividually or simultaneously as required by the operating conditions.

Y further-important object of this invention is to fprovide a combination drive'elevator actuating system f'or ta' lift truck which provides greater heat radiating capacity which results from circulation of the oilthrough -a po'rtion of the'system which is not being operated to eapaity. I

yknother object of this invention is to provide a power s'i-i'pply :and control system for twoseparate functions on a"lift truck, greater e'fiiciency being achieved by' making -thepeakcapacity available to either system.

Other objects and advantages, :more or less ancillary to the foregoing, and the manner in which all the various ob' jects are' realized will appear in the following descriptibn, which, considered in connection with the accompa'n'ying drawings, sets forth the preferred embodiment of the invention.

; -Referring to the drawings: 1

Fig. l is an elevational view of a lift truck utilizing kthe'in'vention disclosed herein; I

Fig. 2 is a schematic diagram of the hydraulic drive system in neutral condition;

i :Fig. 3 is a'schematic diagram of the hydraulic drive system in forward condition;

Fig. 4' is a schematic diagram ofthe hydraulic drive -systejm in the'reversed condition,

Fig. 4a is-adetailed showing of an-alternative arrangernent employing means for unloading one 'of the'pumps in th'e' pressur e sensing valve; I and Fig.-5 is a-schematic diagram of the hydraulic drive system 'in fo rwa'rd creep condition.

The prefer'redembodiment of my invention utilizes a conventional gear type hydraulic pump 10 which is rnou-nte'cl on the r'e'ar end of a lift truck enginelZ. Pump 10 is directly connected to the drive shaft of the engine l2 and rotates therewith to deliver hydraulic fluid tinder pressure whenever the engine is operating. A second pump 14 is connected in tandem with pump 10 {and is of-the unloadin'g type, for example, and as well known i n the 'pump art, the unloading may be accomplished bymai'ntainin'gthe wear plate behind one of the gears -in the pumping :position, and by utilizing an indefp'e'nd'ent' hydraulic pressure control, 'the wear plate may be rnoved toward and away from its gear to accomplish the loadingand unloading thereof. 'A third pump 16 is dr iven in tandem' with pumps 10 and I4 'andha s "the control characteristics of pump 14.

lhehydrairlic system is divided to accomplishcertain requirements of 'thelift truck. The elevating apparatus utilizes at =lea'st 'one cylinder 18 for activating the mast 70 carriage and accessories mounted thereon. A reservoir 20 is "connected through =a 4-way valve 22 to thecylind'er -l8" zind functions as a sump for receiving the fluid trot'zj;

is pumped into or from ports 80, 84 and 74. .3"the plunger is shown in the forward drive position at 3' the cylinders 18 and supplying the pumps 10, 14 and 16. A reservoir 20 is connected by a conduit 24 to a filter 26 which prevents the accumulation of foreign matter in the hydraulic system. A conduit 28 connects the filter 26 and the reservoir 20 to a second reservoir 30 which functions as the main reservoir for the drive system and the pumps 10, 14 and 16. A supply line 32 connects the reservoir 30 with the inlet connection 34 for pump 10,

the inlet connection 36 for pump 14, and the inlet connection 38 for pump 16.

Oil discharged from pump through conduit 40 to a main control valve 42 and is connected to .port 44 therein. The oil discharge from the pump 14 passes through conduit 46 to the 4-way valve 22 which has a high pressure carryover which allows the passage of fluid -therethrough to conduit 48 which is connected through line 50 to a port 52, in the valve 42. The oil discharged from pump 16 passes through conduit 54 to check valve 56 and conduit 50 to port 52. It can be readily seen that the capacity of pump 10 is available at port 44 and the combined capacities of pumps 14 and 16 is available at port 52 when the elevating apparatus is not being operated.

A return line 58 connecting to the reservoir 30 opens into a port 60 in the valve 42. A passage 62 from the port 52 connects through a check valve 64, a passage 66 and a relief valve 68 to the return line 58. A passage 70 connects the port 44 to the passage 66 between the check valve 64 and the relief valve 68. The relief valve .68 prevents over-pressure in the system and the check valve 64 prevents short-circuiting of the hydraulic fluid under certain operating conditions. A passage ,72 connects the port 74 in the valve 42 to the passage 66'which is the low pressure manifold connecting ports 44, 52 and 74.

A reversible hydraulic motor 76 has its output shaft connected to the differential in the driving axle of the lift truck. The hydraulic fluid for operation in the motor 76 is conducted thereto through a conduit 78 which opens into port 80 in valve 42. A return conduit 82 connects theimotor 76 to port 84 in valve 42. It is noted that the flow through conduits 78 and 82 may be reversed by operation of the control valve 42 so that conduit 78'may be either the inlet or the outlet to the motor 76, and conduit 82 may also be either the inlet or the outlet to the motor 76. l

' A plunger 86 is reciprocable to various positions in valve 42 to selectively direct a desired proportion of fluid from the pumps to motor 76 and to direct the remaining proportion of fluid to reservoir 30 through channels bypassing the motor. A plunger 86 is positioned by linkage which is attached to a foot pedal 88 which functions as the manual device operable by the driver of the vehicle to effect the desired change in speed or direction thereof.

In Figure 2 the plunger 86 is shown in the neutral position wherein lands 90 and 92 are located in the bypassports 52 and 44 and away from the walls thereof to allow free fluid flow therein and a shoulder 91 of plunger 86 is disposed to prevent flow of fluid beyond a bypass port 93. In this position of the plunger, free fluid flow is allowed between ports 44,-52 and bypass 93 into a conduit 96, communicating with port 93, and

thence to conduit 58 to the reservoir 30, thus making. it impossible to build up any pressure to cause the hydraulic motor 76 to operate. Under the circumstances no fluid In Figure which point the hydraulic fluid from the outlets of respective pumps 10, 14 and 16 is directed through line third hand. Fluid applied at port 44 is restricted from .flow into chamberWSZ by land 92 .andfis .discharged through line 70 and in sequence flows from line 70 into port 74, into port alonga reduced portion of plunger 86 between land 94 and a shoulder 95 of plunger 86 confining the fluid through conduit 78 and into motor 76. The fluid is discharged from motor 76 into line 82 and flows into port 84, along a reduced portion of plunger 86 into port 60 between lands 94 and 98 and back to reservoir 30 through conduit'58. I

Automatic speed control of-the vehicle is accomplished by a regulating valve 100 which is connected to the pump output line 40 by a connection 102. The fluid connection 102 senses the output pressure of the pump, 10 and supplies this pressure to the port 104 in the valve 100. A plunger 106 is reciprocable in the valve 100 and biased in one direction by a compression spring 108. An adjusting screw 110 predetermines the force applied by the spring 108 to the plunger 106 and allows the valve 100 to be actuated at any desired pressure. The end of the plunger 106-at the'port 104 has the flow pressure applied thereto which is balanced against the spring 108. Ac-

cordingly, when the pressure from the pump discharge reaches a.high value the plunger 106 moves against the spring 108 and opens the valve. 7

A return line 112 connects the ports 114 and 116 in the valve 100 to the reservoir 30. The port 118 is con- ..nected .bys'a line 120 to the pump 16 in such a manner .that the oil under pressure utilized to actuate the pressure plate of pump 16 is vented upon opening of the valve 100. Accordingly, when the plunger 106 is moved by high pressure, oil flows through the line 1 20 to ports 118 and 114 and throughthe line 112 to the reservoir 30. At this condition the pump 16 ceases to discharge fluid "and the output pressure falls to a predetermined normal level? Since only pumps 10 and 14 are operating, the available fluid for operating the motor 76 has been re- -ducedwith the result that the motor vehicle speed is also spring 126 provides a force opposite in directionto the hydraulicpressure and this force is adjustable by positioning the adjusting screw128.

A manual override control valve section is located between the port 124 and the line 130 connecting the valve to the pump14. A plunger 132 having two operating positions allows the operation of the automatic control in one position and in the other position closes the communication between port 124 and the line 130, thus preventing the unloading of pump 14. When the automatic control is in effect, pump 14 may be unloaded and the single pump 10 utilized to furnish the pressure required for driving the vehicle. Accordingly, the speed of the vehicle would be one-third that available when all of the pumps 10,14 and 16 were loaded.

The check valve 56 prevents the return flow of hydraulic fluid through pump 16 when the pumps 10 and 14 are delivering fluid under pressure and pump 16 is unloaded. A'check valve 64 in line 62 also prevents short-circuiting of the hydraulic fluid when pump 10 is the only pump operating to supply pressure to the motor '76. When'the vehicle is rolling downgrade and the motor 76 has a displacement greater than the supply pump capacity, the deficiency of hydraulic fluid is made up through a valve 136 which automatically connects the reservoir 30 to the motor inlet under such condition.

When it is desired to reverse the vehicle the plunger 86 in valve 42 is moved to the right as shown in Fig. 4 by the operation of the pedal 88. In this condition the oil supplied through line 40 is connected through port 44, passage '70 to 'port 74. Due to the position of the plunger port 74 is open to port 84 and the oil is allowed to flow through the valve to line 82 which becomes the pressure supply line tothe motor 76, The line 78 func- {the oilfroni; the motor 76 to thereservoir 3.0. Ther elief gauges protects the system from excessivelyhighpress, by haviug a setting which is slightly lower thanthe mum pressure whiehthe engiue 'could develop when Y- pump is operating. .When theplunge'r, 86 is in motion, the relief valve 68 bypasses oil from line 70 iodi e ,58.thus preventing stalling of ,theengine. With gthis feature the controls may he-rset for a Lchange in d rection of. travel Without first bringing the. vehicle to a top and further, the system may be usedfor breaking hicle byactuating the plunger 86 into a creeping M Q oppos'ite to themotion of thetruck. I w

" In; B g. 4 the automatic, control I valve :is shown with ,xth P Q 2 s .,,,n. .122 their ctu r tionwh c ld're fr m high pump dischargelpressuresin line In each instance the pressureon the pressure, plates e p umps is relieved to the reservoir 30 thus'unloadthe pumps to which theyareconnected. LineflZO t t QQPBJmP. .16 t hrmt h e with P I BJP H v returnline 112 to'.the reservoir .30. .Pump 14 is cgnnected. through lineY13;0 and manual control valve 1 Z p'o rt 124, port 116 to return line 112.- 'Withlhe v 1 s set as. shown Eig rlthevehicle would bein I tsfl au in the p e rah r ihg ump .1 and not be supplying fluid for the operation of the tor; s ince the se pumps are relievedf as above d'e- Revr fiow 9 q i ro u t-n1 .throughithe pumps l t and 16 to thereservoi'r islprevented byland 20 and eheck valve 6,4 in valve 42. Under these condi- Q, 11 id a pl e to al .ft atr r v i ph r fi fr9rh.., h P9 I:1. .n .?I 72in ort 7. 1 )therefmmflows in o po t 8 l n r dus d' rprt o 'nl n's hfi t be a ed h qu hlin v 2 qpump 76in ia revers e sense. In, port 44, 1'ands 90 and'92 restrain 16. flui r9,m. fiqwalbheplung rfi an mp r a 84 lands 9 4 arid??? restrain-the fiuid'fromfiow along the plunger exceptas to thespacetherebetween. The .1 a l syst m nt l iar vh hhs sd to ,P II P. y efifi. When the automatic speed. control unloads both l 9 -t i9 ,th h rauliqs stem Whe it lfie re l .hper'a the l ft n me h nism wh h [,i .trg l d h f 44Way Y1Y .2. e pl n 1321 serv d t919 9 eth Passa 9 o hrou l n 130 frornlpu np which causes the pressureto build up on the pressure pla e -9 P i i ,1 IUnd th nditi p m mmediately begins to function and high pressure isavailable at the 4-way control valve 22. When the control valve 22 is setfor actuating the lift cylindelr, 18, oil flows from line 46 through line 23 to the cylinder 18 and returns through line ZSto-the hydrauIic system reservoir Whe n. -t he hydraulic system,reservoir 20 overflows ,thqoil is,d isc h arged throughithefilter"26 to the, drive inc e m fi t u c ean n the s t m .Acwrdihgly, ;the. manualpverridecontrol preventsrthe unloading of pump 114 ,-thereby ,proyiding a continuous supply of fluid under pressure for actuation. of the lift mechani'smby. the cylinder 18.

"During'the.operationof a lift truck it is desirableno be able to move the vehicle slowly and for short distances which is commonly known as creeping. To accomplish the creeping operation it is necessary to slip a clutch in the conventional vehicle in order to get a small amount of movement and still maintain a reasonable degree of engine speed. Under some conditions a large part of the operation of the lift truck occurs in this manner with the result that maintenance of the clutch assemblies becomes an expensive item. The control system disclosed herein oned .-to j r ever se i thevehicle while the'rv'ehicle is jstill t hath v hi ho; bt njmaximumefliciency of'thetruck it isnecesl -to be able to operate the elevating mechauism hile the vehicle is creeping. .Since the controls and p ply pumps are independent forsthe vehicle drive .and A ing.mechanism actuatorgthe system is particula 'l flexible to accomplishlthis purpose. vIn 5. the linger. $36 of -.,corltrol I valve 42 is shovv n in the position 1O rr sponding-to thecreep condition. This position. of

1th: plunger .86 is obtained by moving the control pedal away from the ,neutral position toward the forward el The {oil discharged from pump 10 through lirie nterst he port 44 which-is connected through passages d 72ito port 74. The port 74 isnow open to port rich supplies oil through line 78 to the motor. 76-. thenliand 912 only partially restricts the flowofoil art-A4 to the return passage 96, the vquantity 0f av labl :to drive the motor.- 7 6 ;is reduced accordof the oil discharged from pump 10, may be directed to i a or 76.:ori tothe r et irn passageyfi and return line e reservoir 30. x

3D continuously.supply. oil under pressure through line "46 to/the flrwayvalve zl whereit available for actuation of hydraulic mechanism 18. The= oil from pump 14 ,w W pa ssesgthrough the vvhigh ,pressure carryover and =l ine.,48. will enter-port .52 and be. discharged through 13 gpassagefifitandline 58 .to the reservoir '30. Operation of thepump 1 4,: vtherefore, does not affect the creepcco'nftrolg characteristicssince no part of the discharge thereis directed to themotorv 7.6.

it -Analternate form of controlfor the pump 14, is shown fhasbeen. inserted. This plug 134 would be. used.- when p mps 1;4 and lfi there is no pressure available in line 9 5 gthle type of .ope ration of the vehicle would make it .un-

. desi-rable fonpump 14 to unload. Theoperatorwould tlnot have. thermejans of controlfor changing the meansfof r peration pump 14. First speed of the vehicle would be obtainedu as previously described, however, the second 5.5. speed.wouldbe obtained by the .full forward or. reverse :positionoflpedal 88. .Third speed would still be autou1 a tic in that pump .16 would still unload at the same :pressure whethenor not'the plug is inv position.

Although the foregoing is necessarily of a detailed 15. ;character, :in. order. that theinvention may becompletely setforth, it is to beuunderstood that the specific terminologyis nottintendedto be restrictive or confining, and

Qthat various.rearrangements of parts modifications ofdetail may beresorted .towithout departing from. the

(Q -scope or spirit of theinventionas herein claimed.

: sai d sec ond pump having aninlet, an outlet andan unloading conduit, said'thir'd'pump having an inlet, an out- I let and an unloading conduit, a reservoir, a conduit connecting each of said pump inlets to the reservoir, pressure sensing valve means comprising first and second pressure responsive valves having pressure sensing chambers, said first pressure responsive valve being connected to the unloading conduit of said second pump, said second presu sure responsive valve being connected to the unloading conduit of said third pump, a conduit connecting the is particularly adapted for the creeping operation since outlet of said first pump to the pressure sensing chamber Qbviduslyby:positioning the plunger 8t5 any part i iBi'der to naintain operation lof purnp ll 4tl forthe 25 supply of oil'fio :th e elevatiugapparatus,during the creeprotary hydraulic motor having an inlet and an outlet, a hydraulic lift cylinder, a four-way valve, a conduit conmeeting the outlet of said second pump to said four-way valve,,a conduit connected to said third pump outlet, a check 'valve in said last-named conduit, a conduit leading from said four-way valve connected to the conduit having the check valve therein, a manually operable control valve, said control valve comprising a valve body having an axial bore therein, a plunger mounted for reciprocation in said bore, said valve body having a plurality of axially spaced annular ports opening into said bore, said plunger having a plurality of spaced lands to open and close adjacent ports with respect to fiuid fiow through the bore, a first port having'conduit means connected to the outlet of saidfirst pump, a second port adjacent said first port, means connecting the combined output from said conduit leading, from said four-way valve and the output of said third pump to said second port, a third portconnected to the inlet of said motor, a fourth port connected to the outlet of said motor, a fifth port between 'said third and fourth ports, said body having a passageway leading from said first and second ports to said fifth port and a land on the plunger in said fifth port to direct the fiu'id fiow from said pumps into said fourth port to drive the motor in one direction or into said third port to drive the motor in an opposite direction, and conduit means connecting said third and fourth ports to the reserlvoir'depending upon the directional drive to the motor.

2. A pump-motor system for hydraulically transmitting power from a plurality of pumps driven in unison .to a rotary hydraulic motor and to a lift cylinder comprising a first pump having an inlet and an outlet, a

pressure responsive valve, a manually operated control valve, said first pump outlet having two branches, a first branch leading to said pressure responsive valve and the other branch leading to said manually operated control valve, 2. second pump having an inlet and an outlet, said outlet being connected to a four-way valve for said lift cylinder, said second pump having an unloading outlet leading to said pressure responsive valve, a third pump having an outlet and an inlet, said last-named outlet havbody having an axial bore therein, a plunger reciprocable in said bore, said bore having a plurality ofraxially spaced ports therein surrounding the plunger, said plunger having a plurality of spaced lands adapted to openand close 'said bore with respect to adjacent ports, a first portconnected to the said other branch of said first pump, a second port, said valve body having an open passageway therein leading from said first port to said second port, said second .port being connected to the inlet of said rotary motor, a third port adjacent said second port, said third port being connected to the outlet of said motor, a fourth port adjacent said first port, said one branch of said third pump being connected to said fourth port, a passageway leading from said fourth port to said second port whereby under low torque at said motor said second and third pumps supplement the drive from the first pump, a land on the plunger between said second and third portswhereby fiuid fiow from the first and fourth ports is directed into either said second port or said third port to control the direction of rotation of said rotary motor, and conduit means connecting said second and third ports to the reservoir depending upon the direction of rotation of said motor. t

3. A hydraulic pump-motor system for transmitting power from a plurality of pumps driven in unison toa motor, a reservoir, a first, second and third pump, each 'of said pumps having an inlet and an outlet, pressure responsive valve means for unloading said second and third pumps to the reservoir, said pressure responsive valve means including a pressure sensing chamber connected to the outlet of said first pump whereby said second and third pumps are unloaded in response to a predetermined high pressure in said sensing chamber and said pumps are loaded in response to a predetermined low pressure in said sensing chamber, manually operated control valve means to direct a portion of the output from said first pump to said motor and by-pass the balance of said output from said first pump to said reservoir, comprising a valve body having an axial bore, a

plunger mounted for reciprocation in said bore, a plurality' of spaced annular ports in said bore surrounding said plunger, a plurality of spaced lands on the plunger to open and close the bore with respect to adjacent ports, a [first port connected to the outlet of said first pump, a second part adjacent the first port and connected to the outlet of said second and third pumps, a fourth port adjacent the ,second port and an opening thereinto, conduit means connecting said fourth port to the reservoir, a third port connected to the inlet of said motor, a fifth port adjacent said third port, said fifth port being connected to said motor outlet and to the reservoir, said valve body having a passageway therein connecting said first, second and third ports, said passageway having a branch leading to said reservoir, a relief valve in said branch, a land onsaid plunger between said third-and fifth ports to direct flow from said first and second ports to said motor .inlet and a land on said plunger between said first and second ports to direct a part of the flow from said first port to said second port and thence to the fourth port and the reservoir, and a land on the plunger between the second and fourth ports adapted to be moved with the plunger to close the fourth port with respect to the second port and thereby direct the output of all three pumps to the motor.

References Cited in the file of this patent UNITED STATES PATENTS 1,716,881 Francis June 11, 1929 1,982,711 Vickers Dec. 4, 1934 2,074,618 Roeder Mar. 23, 1937 2,276,895 Vosseler et al. Mar. 17, 1942 2,320,600 Howell June 1, 1943 2,507,357 Stoner May 9, 1950 2,553,458 Jordan May 15, 1951 2,598,865 Turner June 3, 1952 

